The present invention relates to a method for evaluating a support surface for supporting a body of a patient. More particularly, the present invention relates to a method and apparatus for evaluating interface pressure performance of the support surface. Such an evaluation is beneficial for designers in their efforts to produce better support surfaces that will reduce the incidence of pressure ulcers.
Ten percent of all acute care patients in the U.S. and twelve percent in Europe, suffer from pressure ulcers (decubitus ulcers, bed sores) on any given day. More than half of these patients are over the age of 65. Pressure ulcers are painful for patients and can be very costly to treat. Efforts to reduce the rate of occurrence have mainly been focused on prevention of hospital acquired pressure ulcers.
One preventative approach has been through the improvement of patient support surfaces (i.e. beds, tables, stretchers, chairs, etc.) that minimize external forces (i.e. pressure and shear) on a patient's body. The ability to determine the performance of the support surfaces is required for the design of better surfaces and the interpretation of clinical outcomes. Designers have generally relied upon incomplete anthropometric data and subjective data from patients and clinicians to estimate performance of support surfaces.
Interface pressure, the loading between a patient's skin and the support surface, can be used to determine relative differences in support surface performances. Several different sensor technologies exist for the measurement of interface pressure, each with its particular performance characteristics. See, for example, U.S. Pat. Nos. 5,357,804; 5,253,656; 5,148,706; 4,827,763; 2,644,332; and 2,378,039. There are several common critical parameters for reliable interface pressure measurement including: overall size, flexibility, resolution, accuracy, and repeatability. Each of these parameters affects the reliability of collected data.
The type of data used in the evaluation of performance is equally important as the reliability of the data. The most commonly reported data is single value maximum pressures recorded at several "critical" areas of the body. These maximum pressures provide somewhat useful data, but they do not give any indication of the number of peak pressures, the overall size of the pressure peaks, the average pressure on the entire body, or even maximum peak pressures in locations of the body not considered "critical".
The present invention provides a single repeatable indicator of interface pressure performance that accounts for the magnitude, number and size of all pressure peaks as well as the overall average pressure on the body. A Pressure Index (P.sub.index) has been developed in the present invention to provide a single numerical representation of the effectiveness of a support surface.
Support surfaces are used by a wide range of patients differing in height, weight, morbidity, etc., all of which will affect support surface performance. Since interface pressure results are highly dependent on the particular measurement system used, the test subjects, the head elevations, the testing environment, and the test procedure, it is not valuable to compare maximum pressure values or any data values from sources that do not have consistent equipment and test methods.
The system and standardized method of the present invention reliably and repeatably measures all the interface pressure characteristics, while adequately representing the desired population, and allowing for realistic patient positioning.
Interface pressure measurements are highly sensitive to subject variability and the method of subject placement. No two humans, even of the same weight and stature, are anatomically identical. Therefore, interface pressure data taken on humans can vary a significant amount from person to person and even from test to test. The method used to place the subject on the surface and any movement of the subject also directly influence the interface pressure data.
The test method of the present invention uses anthropometrically correct manikins to minimize subject variability and standardize method of subject placement. The manikins adequately represented at least 90% of the elderly (65-74 years old) US population. Illustratively, manikins representing a 4'10" (1.5 m) and 104 lb (47.2 kg) female, a 6'0" (1.8 m) and 213 lb (96.6 kg) male, a 5'2" (1.6 m) and 135 lb (61.2 kg) female, and a 5'5" (1.65 m) and 300 lb (136.1 kg) female are used for testing each support surface in the method of the present invention. It is understood that other manikin sizes may be used, if desired.
The manikins are constructed using the body height and weight as independent variables, regression equations from two U.S. Air Force studies were used to estimate the physical dimensions and weights of the individual segments of the body. The manikins are constructed using the weight and dimension data for each segment as a blueprint. Each segment is weighed several times throughout the construction process to verify that it is correct.
The manikins include a wooden shell with a steel skeletal system. The joints are created to mimic the human range of motion. For simplicity the motion of the head is limited to flexion extension, no rotation was allowed. Also no medial/lateral rotation of the leg or pronation/supination motion of the arm is provided. The motion of the spine is also limited to flexion/extension, no medial/lateral motion is provided. The spinal column is constructed of 5 interlocking "Y" shaped pin joints in order to simulate the flexibility of the human spine. The shoulder and hip joints are simulated using ball joints, while the ankle, knee, elbow, and neck are simulated using hinge joints. The joints of each manikin do limit the overall range of motion, but not the range of motion necessary to achieve the various positions tested.
The manikins are then covered with a thin layer of latex foam and polyurethane coated fabric. The foam and fabric is designed to simulate skin and subcutaneous fat layers while not introducing any time dependent properties to the manikin.
When reviewing the data of different support surfaces the boundary conditions of the test have practical importance. Typically, the interface pressure data reported is collected with the subject in the supine position (0.degree. head elevation), using a very limited sample size, and with an inadequate representation of the relevant population. The head elevation of the bed has a great impact on performance since it changes the weight distribution on the support surface. The present invention uses a range of head elevations from 0 to 45 degrees which best represents the common positions utilized in the clinical setting for testing the support surfaces of a bed.
Illustratively, interface pressure measurements are taken at three elevations of the head section of the bed at 0, 30, and 45 degrees. The 0 degree head elevation is selected because it reflects a supine position, most commonly used in the hospital for critically injured or ill patients. The 30 degree head elevation is selected because it represents a typical head elevation caregivers place patients in during their recovery to promote clearing of the lungs and aid in healing. Finally, the 45 degree head elevation is selected because it is the highest elevation patients usually used in recovery activities (i.e. eating, watching television, etc.). The beds are raised to these positions using standard operating mode which also articulates the knee section of the bed as the head is elevated.
A gantry of the present invention allows the testing personnel to pre-position the manikins in the appropriate positions for each bed or chair position. It is important that the manikin's body not be drawn across the surface during loading, since this will introduce non-normal forces. The positions of the manikins are selected to allow all body parts of the manikin to contact the surface approximately at the same time, therefore minimizing non-normal loading of the skin of the manikin. The gantry also allows the reproducible placement of the manikins in the same position for every test. During the collection of data the gantry does not carry any of the weight of the manikin.
To test pressure on the manikin, a sensor pad is positioned on the support surface before the support surface moves into engagement with the manikin. Illustratively, the sensor pad detects pressure in over 8000 different nodes on the support surface as discussed below.
The P.sub.index is then calculated for each surface based on the node pressures. The P.sub.index is calculated using a mathematical equation discussed below to evaluate the closeness of a support surface to that of an ideal surface (one with a homogeneously distributed pressure of 10 mmHg across the entire interface area). The average pressure value of nodes having pressure greater than or equal to the optimum pressure as well as the standard deviation of these nodes from the average pressure are used to calculate the pressure index of the present invention.
According to one aspect of the present invention, a method is provided for evaluating pressure interface performance of a support surface for supporting a body. The method includes the step of providing an array of pressure sensors covering substantially the entire support surface. The pressure sensors provide an output signal indicating interface pressure with the body at a plurality of separate pressure nodes on the support surface. The method also includes the step of calculating a pressure index using all of the pressure nodes to generate a single numeric value indicating pressure interface performance of the entire support surface.
According to another aspect of the present invention, a method is provided for evaluating pressure interface performance of a support surface for supporting a body. The method includes the step of providing an array of pressure sensors on the support surface. The pressure sensors provide an output signal indicating pressure at a plurality of pressure nodes on the support surface. The method also includes the steps of aligning the support surface at a selected position, engaging the support surface with the body, determining which pressure nodes have a pressure value greater than an optimum pressure value, and storing the node pressure values greater than or equal to the optimum pressure value. The method further includes the steps of calculating an average of the stored node pressure values, calculating a standard deviation of the stored node pressure values from the average pressure value, and calculating a pressure index for the support surface based upon the average value, the standard deviation, and the optimum pressure.
In the illustrated method, the step of engaging the support surface with the body includes the steps of suspending a manikin over the support surface and moving the support surface upwardly into engagement with the manikin. The step of suspending the manikin over the support surface includes the steps of adjusting the position of the manikin to a selected position relative to the support surface based upon coded indicators formed on a support coupled to the manikin.
According to yet another aspect of the present invention, an apparatus is provided for evaluating pressure interface performance of a support surface for supporting a body. The apparatus includes an array of pressure sensors located on the support surface. The pressure sensors provide an output signal indicating interface pressure with the body at a plurality of different pressure nodes on the support surface. The apparatus also includes a processor coupled to the output signal from the array of pressure sensors. The processor includes means for generating a pressure index for the support surface based upon sampling all the plurality of pressure nodes to generate a numerical representation of the pressure interface performance for the entire support surface.
According to still another aspect of the present invention, an apparatus is provided for evaluating pressure interface performance of a support surface for supporting a body. The apparatus includes an array of pressure sensors located on the support surface. The pressure sensors provide an output signal indicating interface pressure with the body at a plurality of different pressure nodes on the support surface. The apparatus also includes a processor coupled to the output signal from the array of pressure sensors. The processor includes means for determining which pressure nodes have a pressure value greater than an optimum pressure value, for storing the node pressure values greater than or equal to the optimum pressure value, for calculating an average of the stored node pressure values, for calculating a standard deviation of the stored node pressure values from the average pressure value, and for calculating a pressure index for the support surface based upon the average value, the standard deviation, and the optimum pressure.
According to a further aspect of the present invention, an apparatus is provided for supporting a manikin in at least two different orientations for testing a support surface. The apparatus includes a gantry including a top frame, a plurality of connectors coupled to the manikin, and a plurality of fasteners configured to couple the connectors to the frame at selected positions for each of the at least two orientations of the manikin. The apparatus also includes first coded indicators on the frame for indicating a position for each of the connectors relative to the frame corresponding to the at least two different orientations of the manikin, and second coded indicators on each connector for indicating a required length of the connectors to position the manikin in each of the at least two orientations.
In one illustrated embodiment, the first and second indicators are color coded indicators, with a separate color representing each of the at least two orientations of the manikin. In another illustrated embodiment, the first and second coded indicators are numeric indicators, with a separate number representing each of the at least two orientations of the manikin.
Also in the illustrated embodiment, the connectors are chains having a plurality of links. The second coded indicators are coupled to predetermined links of the chain to indicate the required length for the chains to position the manikin in each of the at least two orientations. The manikin is adjustable to at least three separate positions corresponding to a support surface having a 0 degree head elevation, a 30 degree head elevation, and a 45 degree head elevation.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.